The present invention generally relates to dual alloy turbine wheels and, more particularly, to spun metal forms used to manufacture dual alloy turbine wheels.
Turbine wheels comprising two distinct alloys have been used extensively in gas turbine engines. Dual alloy wheels have been used to address the need for hubs and castings having different material properties. Dual alloys have been used to provide turbine wheel hubs having one set of material properties and turbine wheel castings having another set of material properties. Turbine wheel hubs have been formed from alloys that have high tensile strength and low cycle fatigue resistance. Turbine wheel castings, which are exposed to the higher temperatures of the gas path and higher centrifugal loads, have been formed from alloys that have high stress rupture and creep resistance. The two dissimilar alloy parts have been joined by hot isostatic pressing to form dual alloy turbine wheels.
Hot isostatic pressing (HIP) utilizes an autoclave and a pressure transfer medium, such as inert argon gas, to facilitate diffusion bonding of the two dissimilar metals. Vacuum sealing the interface between the casting and the hub is necessary for acceptable diffusion bond formation. Metal or ceramic shaped containers have been used to completely enclose and vacuum seal the dual alloy components during HIP. Unfortunately, these methods are unsuitable for some applications due to container leakage and geometric limitations.
Other methods for producing dual alloy turbine wheels by HIP have been disclosed in U.S. Pat. No. 4,581,300. In this method, the casting and hub are assembled. A sealing plate is then electron-beam welded and vacuum brazed to the casting. Although this method may be used to vacuum seal the interface, braze alloy contamination of the interface is common. Braze alloy contamination in the structural region of the part is unacceptable in some applications and results in poor field performance. Using these methods, scrap due to braze alloy contamination has been reported to be about 20% and the associated manufacturing cost to be about $500,000/year.
Another HIP method is described in U.S. Pat. No. 4,603,801. In this method, the pressure transferring medium comprises a granular glass medium. The interface is isolated from the pressure transferring medium by a stainless steel interference fit seam isolator. Although braze alloy contamination of the dual alloy interface may be prevented by using these methods, the disclosed processes are not useful for many applications.
Another method for preventing braze alloy contamination is disclosed in U.S. Pat. No. 4,796,343. In this method, annular braze traps are used to prevent braze contamination of the interface. Braze trap formation requires machining of the hub and casting. Unfortunately, the machining necessary to form the braze traps is expensive and exacting. Because the casting is brazed to the hub, the re-working of leaking assemblies is not possible and further increases production costs.
An expendable spun metal form capable of preventing braze alloy contamination is needed. Also, there is a need for improved methods of preventing braze alloy contamination of a dual alloy interface. A method is needed wherein braze trap machining is not necessary. Moreover, there is a need for a method wherein the hub and the casting of a leaking assembly can be re-worked.